A recent development in the design of solid-state photodetectors is the silicon photomultiplier (SiPM) [1]-[5], which is capable of single-photon detection and operation inside magnetic fields. SiPMs have been considered as an alternative to photomultiplier tubes in many applications ranging from radionuclide imaging in single-photon emission computed tomography (SPECT) and positron emission tomography (PET) to particle physics, nuclear physics, and astrosphysics. SiPMs are also known by many other names: metalresistor-semiconductor APD, micropixel APD, multipixel photon counter, Geiger-mode APD, solid-state photomultiplier, etc. Many research groups and commercial manufacturers have developed specific configurations of SiPMs.
A SiPM is a large number of small identical avalanche photodiodes (APDs) (cells or microcells) operating in Geiger mode arranged in a matrix. A microcell of a SiPM has dimensions typically ranging from about 10 microns to 100 microns. Each microcell operates as an independent photon counter in Geiger mode; the operating voltage is biased about 5% to 20% above the breakdown voltage. A photon impinging on one microcell can create free carriers that can give rise to a Geiger-mode discharge. This discharge is quenched when the microcell's voltage drops below the breakdown voltage when the discharge current passes through an integrated quenching resistor. The microcell is a binary device since the signal from a microcell has approximately the same shape and amplitude. The discharge currents from all microcells are added on a common load resistor; therefore, the output signal of a SiPM is the sum of the signals from all the microcells firing at the same time.
Key features of SiPMs include high gain (105 to 106), low bias voltage (<100 V), insensitivity to magnetic fields, good timing resolution, and low power consumption. Further, SiPMs can be fabricated using complementary metal-oxide-semiconductor (CMOS) technology, which can potentially reduce the cost of these devices.